iOS 15.1 needed for Apple Watch 7

Phone will not update

Check 

·       you are on a good Wi-Fi, 

·       on charge with more than 50% charged, 

·       no VPN installed 

·       low power mode turned off. 

·       Fully working network connection via SIM.

·       Check Settings, General to make sure you have no Beta profile installed.

Resources:-

Update phone

Update your iPhone, iPad, or iPod touch - Apple Support

Phone will not update

If your iPhone, iPad or iPod touch won’t update - Apple Support

User Tip updating iOS

 Updating iOS: common issues and solutions - Apple Community

Use a computer.

Are you sure you don't have VPN?

Restart the phone.

Did you try all the alternative update methods in the article?